1893.] MICROSCOPICAL JOURNAL. 333 



if it is more convenient, the same result may be obtained by 

 tiikinp one negative with the slide Hat on the stage and the other 

 witii one .side or one end of tlie .slide raised. But if only oneside 

 or end is raised, it must be raised twice as much as each side or 

 end would have to be if raised alternately. With objectives of 

 from ont' to one-fifth inch focus, the end.s of a slide should be 

 raised alternately a little less than one-eighth of an inch ; or if 

 the axis of tilt is to be parallel with the long axis of the slide, 

 the edge of the slide should be raised a little less than one-six- 

 teenlh of an inch. 



When but one-half of the objective is used, one exposure is 

 made with one-half the front or back of the objeclives covered 

 and the second exposure is made with the opposite half covered. 

 In mounting the pictures from these negativ-s, their upright 

 axis must be parallel with the surface of the hemisphere of the 

 objective and as the microscopic image is inverted, the picture 

 taken with the right half of the lens must be mounted so that it 

 will be viewed by the left eye and that taken with the left half 

 with the right eye. In mounting pictures taken with the slide 

 tilted, the one taken with the right end of the slide raised cor- 

 responds to that taken with the right half of the objective and 

 should be mounted to be viewed by the left eye. If the pic- 

 tures are inversely mounted, pseudoscopic, instead of stereo- 

 scopic, effect will be produced. The pictures should not be more 

 than two and three-fourth inches in width. They must corres- 

 pond ; and that they may do so, a good way is to place the two 

 pictures together, and while viewing them by transmitted light 

 so placed one above the other that the like parts of the pictures 

 accurately cover each other, then while they are held in this 

 position they can be cut together to the required size. 



The range of the usefulness of stereoscopic photo-micrography 

 is mainly below amplifications of 300 diameters, but witliin this 

 limit there is large opportunity for its advantageous use in real- 

 istic, if the term may be used, photographic representation of 

 many objects of scientific interest. 



Brain Matter — The nervous matter of the brain and spinal 

 column appears to the anatomist and to the microscopist as ex- 

 actly alike in every healthy human being but it is believed that 

 there must be invisible differences corresponding to the differ- 

 ence between the mind of Gladstone and that of a mere beggar. 



